1. Field of the Invention
This invention relates to electronic engine control of internal combustion engines.
2. Prior Art
it is known to control the air/fuel ratio (A/F) of internal combustion engines using exhaust gas oxygen sensors positioned in the exhaust stream from the engine and an electronic control module coupled to the exhaust gas sensor. Because of the response time of this system and such components as catalysts in the exhaust gas stream, there are occasions when erratic low frequency oscillations occur with feedback from EGO sensors placed after the catalyst. It would be desirable to eliminate such erratic low frequency oscillations.
It is known to have A/F feedback systems for engines with exhaust gas oxygen (EGO) sensors placed behind the catalyst in an effort to achieve more precise A/F control with respect to the catalyst window. The rationale for this action is illustrated in FIG. 1A and 1B which show catalyst conversion efficiency and EGO sensor output voltage versus A/F irrespectively for sensors located both in front of and behind a typical catalyst. As this figure indicates, the switch point of the precatalyst EGO sensor does not coincide exactly with the catalyst window, whereas the switch point of the post-catalyst sensor generally does.
Unfortunately, closed-loop A/F control systems using feedback from a post-catalyst EGO sensor frequently display erratic low-frequency oscillations under certain operating conditions. Two examples of this are illustrated in FIGS. 2A and 2B which show plots of post-catalyst EGO sensor output voltage versus time obtained when the engine was operated under closed-loop A/F control using conventional low-gain integral feedback from the post-catalyst EGO sensor. In FIG. 2A, the EGO sensor output voltage shows an erratic low-frequency oscillation of approximately 0.024 Hertz, while in FIG. 2B, the sensor output voltage shows a well-defined oscillation of approximately 0.015 Hertz. Such low-frequency oscillations are somewhat unpredictable, and occur with certain combinations of catalysts and EGO sensors, but not with all. These low-frequency oscillations are undesirable both from an emissions standpoint (because they produce a loss of catalyst conversion efficiency) and from a catalyst monitoring standpoint (because they can cause erroneous indications from the catalyst monitoring system). These are some of the problems this invention overcomes.